Copolymers of monocyclic esters and carbonates and methods for making same

ABSTRACT

Copolymers having repeating units derived from monocyclic esters or carbonates and certain bicyclic diesters and/or carbonates have controllable rheological properties. The bicyclic diester and/or carbonate copolymerizes easily with the monocyclic monomers, especially with lactide, to form copolymer that can have tailored levels of branching. The copolymers have excellent rheological properties, including increased melt tensions and improved shear thinning, compared to the analogous linear polymers.

BACKGROUND OF THE INVENTION

[0001] This invention relates to polyesters and/or carbonates that havemodified rheological properties and methods for making those polyesterand/or polycarbonates.

[0002] Certain monocyclic compounds can be polymerized to formpolyesters or polycarbonates. Examples of those monocyclic estersinclude dioxanones (such as p-dioxanone), lactones (such asε-caprolactone or 4-valerolactone), dioxan(dione)s (such as glycolide,lactide or tetramethyl-1,4-dioxan-2,5-dione), carbonates such asethylene carbonate and trimethylene carbonate, and ester-amides (such asmorpholine-2,5-dione). Commercial interest in these polymers,particularly polylactide polymers (also known as polylactic acid, orPLA), is rapidly increasing. Unless modified in some way, thesepolyesters are linear molecules and therefore thermoplastic materials.They are useful for making a variety of films, fibers and otherproducts. In the case of PLA, these polymers offer the significantadvantages of being derived from renewable resources (lactic acid can beprepared from plant carbohydrates such as dextrose) and of beingbiodegradable. However, the rheological properties of these polymers aresuch that they can be difficult to process in certain applications. Thisdifficulty in processing has so far limited the applications for whichthese polymers can be used. For example, in extrusion coating, poorrheological properties lead to phenomena such as neck-in and drawinstability (draw resonance and edge weave). Poor rheological propertiesmake it very difficult to make blow molded articles at all, and causeextruded foams to collapse because operating windows are extremelynarrow.

[0003] The rheological property of primary interest is often meltelasticity, which is often expressed as “melt strength”. Broadlyspeaking, it is desirable that a thermoplastic polymer forms a melthaving a reasonably low shear viscosity so that it can be processedreadily, but at the same time the molten polymer must possess enoughstrength that, once formed into a desired shape, it can hold that shapeand in some instances even be worked with until it has time to cool andharden. As a general rule, melt strength can be increased in athermoplastic resin by increasing the molecular weight. However, thisalso increases the shear viscosity so that the benefits of improved meltstrength are offset by the increased force that is needed to shape thepolymer in the first place. The increased force needed requires, atminimum, higher power consumption to process the polymer. In some casesthis means that heavier, more expensive equipment is needed, or elseprocessing rates must be reduced. In addition, increasing molecularweight tends to increase the processing temperatures that are required,and this exacerbates polymer degradation.

[0004] Accordingly, attempts to improve the processing characteristicsof these polymers have tended to focus on introducing branching throughsome mechanism. In the case of PLA, for example, it has been attemptedto copolymerize lactide with an epoxidized fat or oil, as described inU.S. Pat. No. 5,359,026, to treat PLA with peroxide, as described inU.S. Pat. Nos. 5,594,095 and 5,798,435, and to use certainpolyfunctional initiators as described in U.S. Pat. Nos. 5,210,108 and5,225,521 to Spinu, GB 2277324 and EP 632 081.

[0005] Unfortunately, none of these methods is entirely satisfactory. Insome cases, the rheological properties of the polymer are not improvedas much as desired. Good rheological improvements can be obtained inother cases but the manufacturing process is difficult to control, whichmakes it difficult to make the desired product in a reproducible way.Sometimes, the branching agent does not copolymerize well with themonocyclic ester or carbonate. This is particularly true in the case oflactide. In still other cases, the steps required to induce branchingcan interfere with the polymerization. This can lead to increasedpolymerization times, uneven product quality, and other problems.

[0006] It would be desirable to provide a polymer of a monocyclic ester(or corresponding hydroxy acid) and/or monocyclic carbonate, whichpolymer has improved rheological properties, yet remains processable attemperatures below that at which the polymer begins to degradesignificantly. Biodegradability would be a further advantage. It isfurther desirable to provide a convenient process by which monocyclicesters and/or carbonates can be polymerized to form polymers havingimproved rheological properties, and in particular a process which iseasily controllable to form polymers having predictable and reproduciblerheological properties.

SUMMARY OF THE INVENTION

[0007] In one aspect, this invention is a copolymer having, inpolymerized form, units derived from (a) a monocyclic ester orcorresponding hydroxy acid or (b) a monocyclic carbonate, or both (a)and (b), and units derived from a bicyclic diester and/or carbonate.

[0008] Depending on the proportion of bicyclic diester and/or carbonateunits the copolymer may range from slightly branched to denselycrosslinked. Branching in these copolymers tends to be long-chain typebranching, as described more below. In preferred embodiments, theproportion of bicyclic monomer units is such that the copolymer is athermoplastic that exhibits excellent melt strength yet is readily meltprocessable. The preferred copolymers exhibit, for example, reducedneck-in and improved web stability when processed in extrusion coating,compared to the corresponding linear polyesters or polycarbonates, andare more easily processed in blow molding and extrusion foamingoperations.

[0009] In another aspect, this invention is a method comprisingsubjecting a mixture including a monocyclic ester and/or carbonate and abicyclic diester and/or carbonate to conditions sufficient to polymerizethe mixture to form a copolymer having, in polymerized form, unitsderived from the monocyclic ester and/or carbonate and units derivedfrom the bicyclic monomer.

[0010] This method provides a convenient, reproducible way to makecopolymers that have a controlled amount of branching, as the extent ofbranching increases with increasing proportions of the bicyclic diesterand/or carbonate. In the case where the monocyclic monomer is lactide,the polymerization reaction proceeds predictably, controllably and atnearly the same rates as lactide homopolymerizations under the sameconditions. In the preferred embodiments in which a branched butnoncrosslinked copolymer is desired, the process permits such acopolymer to be prepared with excellent control over its rheologicalproperties and with minimal gelling.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a graph showing the relationship between intrinsicviscosity and molecular weight for certain embodiments of copolymers ofthe invention.

[0012]FIG. 2 is a graph showing dynamic mechanical spectroscopy data forcertain embodiments of copolymers of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] For the purposes of this invention, the terms “polylactide”,“polylactic acid” and “PLA” are used interchangeably to denote polymershaving the repeating lactic acid units as described above, irrespectiveof how those repeated units are formed into the polymer.

[0014] The copolymer contains polymerized units derived from (a) amonocyclic ester or the corresponding hydroxy acid or (b) a monocycliccarbonate. For purposes of this invention, “monocyclic esters” includeany monocyclic molecules that contain one or more ester linkagesincorporated into the ring structure, and which are polymerizable.Similarly, “monocyclic carbonates” are any monocyclic molecules thatcontain one or more carbonate linkages incorporated into the ringstructure, and which are polymerizable. Monocyclic esters (and thecorresponding hydroxy acids) and monocyclic carbonates are referred toherein collectively as “monocyclic monomers”. Examples of suitablemonocyclic esters and carbonates include lactones such as ε-caprolactoneor 4-valerolactone; dioxanones such as p-dioxanone; dioxan(dione)s suchas glycolide, lactide or tetramethyl 1,4-dioxan-2,5-dione; carbonatessuch as ethylene carbonate and trimethylene carbonate; and ester-amidessuch as morpholine-2,5-dione. The hydroxy acids are hydroxyl-substitutedcarboxylic acids equivalent to those formed by hydrolyzing the estergroup(s) of the corresponding monocyclic esters. The hydroxy acidsinclude alpha-, beta-, gamma, and epsilon-hydroxy carboxylic acids suchas glycolic acid, dimethyl glycolic acid, lactic acid,epsilon-hydroxycaproic acid, alpha-hydroxy valeric acid, and the like.The copolymer may contain polymerized residues of two or more of thesemonomers, in block and/or random arrangement. Polylactide (PLA)copolymers are most preferred.

[0015] The copolymer also contains units derived from a bicyclic diesterand/or dicarbonate (sometimes referred to herein collectively as“bicyclic monomers”). The units derived from the bicyclic monomerintroduce branching sites into at least some of the copolymer molecules.The structure of the polymerized bicyclic monomer units is derived fromthat of the bicyclic monomer, which is a bridged cyclic compound havingtwo ester and/or carbonate groups in a ring structure and at least oneatom in the bridge. The bicyclic monomer can be represented as:

[0016] wherein each R is independently hydrogen, alkyl or inertlysubstituted alkyl, each n and each o is independently zero or a positiveinteger, and Y is a bridging group. Z is a covalent bond in the case ofan ester group and —O— in the case of a carbonate group. R is preferablylower (C₁₋₄) alkyl and is most preferably hydrogen. Y is suitably—CR₂)_(m)—, where m is a positive integer, —O—, —S—, —NR¹— (in which R¹is alkyl or substituted alkyl) and the like or a combination of two ormore of these groups. Each n is preferably no greater than 1. Each o ispreferably zero. The values of each n and o, taken together, are morepreferably so that the main ring (including the two ester and/orcarbonate groups but excluding the —Y— bridge) contains 6 or 7 membersin the case of esters and 8 or 9 members in the case of carbonates. Mostpreferably, each n is zero and each o is zero. Y is preferably—(CR₂)_(m)— where m is at least one, preferably 1, 2 or 3, and mostpreferably 2. Each Z is preferably a covalent bond, making the bicyclicmonomer a diester. The most preferred bicyclic monomer is2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione. The preferred, more preferredand most preferred values of n, o, Y and m impart to the bicyclicmonomer a reactivity similar to that of lactide, and thus enhance theability of the bicyclic monomer to copolymerize with lactide.

[0017] Bicyclic monomers in which o is zero and Y is —(CR₂)_(m)— can beformed from substituted dicarboxylic acids of the form

[0018] wherein R, Z, m and n are as defined before, and X is a groupthat will react with a carboxylic acid group to form a covalent bond tothe carbonyl carbon, or a group that is displaced by the carboxylic acidgroup. X is preferably hydroxyl, halogen or —NO₂, and is more preferablychlorine or bromine. Substituted dicarboxylic acids of structure II canbe prepared from the corresponding unsubstituted diacid chloride byreacting it with an agent that will introduce the X groups, and thenreacting the resulting substituted diacid chloride with water ifnecessary to convert the acid chloride groups to free acid form. Thebicyclic diester is then generated by heating the substituteddicarboxylic acid in the presence of a weak base such as sodiumcarbonate. Synthesis methods of this type are described, for example, byH. LeSueur, “The Action of Heat on α-hydroxycarboxylic Acids, Part IV,racemic α,α′-dihydroxyadipic acid and meso-α,α′-dihydroxyadipic acid”,J. Chem. Soc. 1908, 93, 716-725; R. Kostyanovskii et al. in “Theautofitting of dilactones from the d,l-forms ofα,α′-dihydroxy-α,α′dimethylglutaric acid andα,α′-dihydroxy-α,α′-dimethyladipic acid”, Bull. Acad. Sci. USSR Div.Chem. Sci. (Eng. Transl.) 1986, 35, 2420-2421; R. Kostyanovsky et al.,“Autoassembly of cage structures 5:, Synthesis, stereochemistry andcyclization of α,α′-dihydroxy-α,α′dimethyladipic acid derivatives”,Russian Chemical Bulletin, 1994, 43(4) 599-607; and Kostyanovsky et al.,“Autoassembly of cage structures 9*, complete autoassembly of dilactonesof α,α′dihydroxy-α,α′-dialkoxycarbonyladipic and -pimelic acids”,Russian Chemical Bulletin 1995, 44(2) 318-321, all incorporated hereinby reference.

[0019] Although the invention is not limited to any theory, it isbelieved that the bicyclic monomer reacts during polymerizationconditions to open one of the ester or carbonate groups in the main ringto form a polymer containing a cyclic ester or carbonate group in thepolymer chain, represented by the structure:

[0020] wherein O—M—C(O)— represents a polymerized unit of a monocyclicester (or corresponding hydroxy acid) or carbonate monomer and prepresents a positive number. The cyclic ester or carbonate group in thepolymer chain can then undergo a further ring-opening reaction withadditional monocyclic monomer (or hydroxy acid) to form a branch pointhaving the structure

[0021] wherein q represents a positive number.

[0022] As a result, it is believed that each unit of bicyclic monomerthat undergoes this full sequence of reactions becomes incorporated intoa copolymer molecule and creates a branch point where four polymer“arms” are joined. A more highly branched polymer molecule can be formedif more than one bicyclic monomer molecule is polymerized into thepolymer chain. Depending on the proportion of bicyclic monomer that isused in making the copolymer, not all polymer molecules may contain abicyclic monomer unit incorporated into them. In that case, thecopolymer is in fact a mixture of linear polymers of the monocyclicmonomer and branched copolymers that contain branch points derived fromthe bicyclic monomer. The latter case is generally true with thepreferred thermoplastic copolymers.

[0023] The degree of branching in the copolymer depends on the amount ofbicyclic monomer that is incorporated into it and the molecular weightof the copolymer. At a given molecular weight, increased bicyclicmonomer use increases branching and can lead to crosslinking. The effectof lowering molecular weight is to permit the use of higher proportionsof the bicyclic monomer without causing crosslinking. By varying theamount of incorporated bicyclic monomer and the molecular weight,branching can be controlled so that copolymers are produced having verylight branching, heavier branching or even crosslinking. In this way,the rheological properties of the copolymer can be “tailored” to meetthe processing demands of specific applications. To introduce branchingbut avoid significant crosslinking, the incorporated bicyclic monomeradvantageously constitutes from about 0.05 to about 1.5 percent byweight of the polymer, although these amounts may increase or decrease,respectively, as the copolymer molecular weight is decreased orincreased. Preferred amounts of the incorporated bicyclic monomer willvary according to the processing demands of particular applications.When light branching is desirable, the incorporated bicyclic monomerpreferably constitutes from about 0.1 to about 0.3 percent of the weightof the polymer. To further modify the rheological properties of thecopolymer, from about 0.3 to about 1.0 percent of the bicyclic monomeris incorporated into it, on the same basis. It has been found thatthermoplastic PLA copolymers containing 0.3 weight percent or moreincorporated bicyclic monomer often exhibit melt tensions exceeding 4 cNand even in the range of 6-16 cN, as determined by the method describedin the Examples below. Values such as these are quite high for a PLAresin, and correlate to substantially improved processability in manyapplications.

[0024] A copolymer containing more than about 1.5% of incorporatedbicyclic monomer will typically be crosslinked, although the preciseamount of bicyclic monomer needed to induce crosslinking will dependsomewhat on copolymer molecular weight. Depending on the degree ofcrosslinking that might be wanted for a particular application, theamount of incorporated bicyclic monomer may be as high as 99% by weight,but is more preferably no greater than about 50% by weight, morepreferably no greater than about 15% by weight, and most preferably nogreater than about 10% by weight.

[0025] The copolymer may also contain residues from one or moreinitiator compounds. These initiator compounds may be intentionallyadded to further refine the molecular weight and/or rheologicalproperties of the copolymer or, as is particularly true in the case oflactide, are present as impurities in the monocyclic monomer, and reactduring the copolymerization process to initiate polymer molecules.

[0026] If the bicyclic monomer contains impurities, those impurities mayalso act as initiator compounds. Thus, it is preferred to either purifythe bicyclic monomer (such as to reduce the level of impurities to <5wt. %, preferably <2 wt. %, especially <1 wt. %), or to determine thenumber and type of impurities and take those impurities into account inmanufacturing the copolymer.

[0027] The copolymer may further contain repeating units derived fromother monomers that are copolymerizable with the monocyclic monomer,such as alkylene oxides (including ethylene oxide, propylene oxide,butylene oxide, tetramethylene oxide, and the like). Repeating unitsderived from these other monomers can be present in block and/or randomarrangements. It is preferred that any such comonomer does not introducebranching points into the copolymer, as this makes it more difficult tocontrol its rheological properties.

[0028] The thermoplastic copolymers advantageously have a number averagemolecular weight of from about 10,000, preferably from about 30,000,more preferably from about 40,000 to about 500,000, preferably to about300,000, more preferably to about 250,000, as measured by the GPC/DVtechnique described in the Examples. The thermoplastic copolymersadvantageously exhibit a polydispersity index (PDI, defined as the ratioof weight average molecular weight to number average molecular weightper the GPC/DV technique) of at least about 1.9, preferably at leastabout 2.1, more preferably at least about 2.5, to about 5, preferably toabout 4, more preferably to about 3.5. They advantageously exhibit a dieswell of at least about 1.05, preferably at least about 1.2, morepreferably at least about 1.4 and especially from about 1.5, to about2.0, preferably to about 1.8, when measured under the conditionsdescribed in the Examples.

[0029] Preferred copolymers exhibit a melt tension, measured asdescribed in the Examples, of at least about 0.8 cN, preferably at leastabout 2 cN, more preferably at least about 4 cN, even more preferably atleast about 6 cN, and most preferably at least about 12 cN, to about 16cN or more. It is especially preferred that the copolymers have melttensions within the foregoing ranges while simultaneously exhibitingmelt flow rates, measured as described in the Examples, in the range of1 to about 15 g/10 min, especially 4-12 g/10 min.

[0030] A preferred non-crosslinked copolymer will exhibit a die swell ofat least about 1.1, preferably at least about 1.5, measured as describedin the examples below. The preferred non-crosslinked copolymer will havea ratio of M_(z+1)/M_(n) (measured as described in the followingexamples) of at least about 8, more preferably at least about 10, evenmore preferably at least about 15.

[0031] A preferred method of making the copolymers of the invention isthrough a copolymerization of the bicyclic monomer and a monocyclicester and/or carbonate. In general, polymerization methods andconditions suitable for homopolymerizing the monocyclic esters andmonocyclic carbonates can be used without significant modification,other than the inclusion of the bicyclic monomer into the reactionmixture. Suitable lactide polymerization processes are described in U.S.Pat. Nos. 5,247,059, 5,258,488 and 5,274,073 to Gruber et al; U.S. Pat.No. 5,288,841 to Bellis et al.; U.S. Pat. No. 2,951,828; and U.S. Pat.No. 5,235,031 to Drysdale et al., all incorporated herein by reference.Methods for polymerizing other monocyclic monomers are described in U.S.Pat. No. 5,288,841 to Bellis et al. and U.S. Pat. No. 5,225,521 toSpinu. The bicyclic monomer can be added by mixing it with othermonomers, by feeding it to the reaction apparatus as a separate stream,by adding it as a solution in a suitable solvent, or any otherconvenient way.

[0032] A particularly suitable process for preparing PLA is described inU.S. Pat. Nos. 5,247,059, 5,258,488 and 5,274,073. This process iseasily adapted to make the copolymers of this invention. In the processdescribed in those patents, lactide is fed as a liquid directly to apolymerization system, where it is polymerized at elevated temperaturein the presence of a catalyst. As molecular weight increases, anequilibrium is established between the polymer and free lactide, thuslimiting the build-up of molecular weight and producing a polymercontaining a certain amount of free lactide. The free lactide providessome plasticizing effect that is often undesirable, and also tends tocoat the surfaces of polymer processing equipment. For these reasons,the polymerization process typically includes a devolatilization stepduring which the free lactide content of the polymer is reduced,preferably to less than 1% by weight, and more preferably less than 0.5%by weight.

[0033] The polymerization can be conducted batch-wise, semi-continuouslyor continuously. Continuous stirred-tank reactors and tube or pipereactors are suitable types of polymerization vessels. A series of CSTRsor tube or pipe reactors may be used to conduct the polymerization instages. This permits additives to be introduced at specific stages inthe polymerization process if desired, and also allows for differentreaction conditions to be used at different stages of thepolymerization.

[0034] Suitable polymerization temperatures preferably are, forsolventless processes, above the melting temperature of the monomer ormonomer mixture and above the melting temperature of the productcopolymer, but below the temperature at which significant polymerdegradation occurs. A preferred temperature range is from about 100° C.to about 220° C. A more preferred temperature range is from 120° C. toabout 200° C. and especially from about 160° C. to about 200° C.Residence times at polymerization temperatures are selected to produce acopolymer of the desired molecular weight and/or desired conversion ofmonomers.

[0035] Molecular weight and conversion are controlled by polymerizationtime and temperature, the equilibrium between free lactide and thepolymer, and by the use of initiator compounds. In general, increasingquantities of initiator compounds on a molar basis will tend to decreasethe molecular weight of the product polymer. Unless they are stringentlypurified, monocyclic monomers such as lactide tend to containhydroxy-functional and/or acid-functional impurities that act asinitiators during the polymerization process. If desired, additionalinitiator compounds can be added to provide additional control overmolecular weight. Suitable such initiators include, for example, water,alcohols, glycol ethers, and polyhydroxy compounds of various types,such as ethylene glycol, propylene glycol, polyethylene glycol, polypropylene glycol, glycerine, trimethylolpropane, pentaerythritol,hydroxyl-terminated butadiene polymers and the like.

[0036] A wide variety of polymerization catalysts can be used, includingvarious tin compounds such as SnCl₂, SnBr₂, SnCl₄, SnBr₄, SnO, organotincompounds such as tin (II) bis(2-ethyl hexanoate), butyltin tris(2-ethylhexanoate), hydrated monobutyltin oxide, dibutyltin dilaurate,tetraphenyltin and the like; PbO, zinc alkoxides, zinc stearate,organoaluminum compounds such as aluminum alkoxides, organoantimonycompounds such as antimony triacetate and antimony (2-ethyl hexanoate),organobismuth compounds such as bismuth (2-ethyl hexanoate), calciumstearate, magnesium stearate, certain yttrium and rare earth compoundssuch as are described in U.S. Pat. No. 5,208,667 to McLain et al, andthe like. Catalysts are used in catalytically effective amounts, whichdepend somewhat on the particular catalyst, but are usually in the rangeof from about 1 mole catalyst to about 3000-50,000 moles monomers.Preferred catalyst concentrations are in excess of 5000 moles monomersper mole catalyst, and especially in excess of 10,000 moles monomers permole catalyst. The catalyst may be supported if desired to facilitateits removal.

[0037] In order to produce a melt-stable lactide polymer, it ispreferred to remove or deactivate the catalyst at the end of thepolymerization process. This can be done by precipitating the catalystor preferably by adding an effective amount of a deactivating agent tothe polymer. Catalyst deactivation is suitably performed by adding adeactivating agent to the polymerization vessel, preferably prior to thedevolatilization step. Suitable deactivating agents include carboxylicacids, of which polyacrylic acid is preferred; hindered alkyl, aryl andphenolic hydrazides; amides of aliphatic and aromatic mono-anddicarboxylic acids; cyclic amides, hydrazones and bishydrazones ofaliphatic and aromatic aldehydes, hydrazides of aliphatic and aromaticmono- and dicarboxylic acids, bis-acylated hydrazine derivatives,phosphite compounds and heterocyclic compounds.

[0038] Certain hydroxy acids, particularly α-hydroxy acids such aslactic acid, exist in two optical isomers, which are generally referredto as the “D” and “L” isomers. Either D- or L-lactic acid can beproduced in synthetic processes, whereas fermentation processes usuallytend to favor production of the L isomer. Lactide similarly exists in avariety of isomeric forms, i.e., “L-lactide”, which is a dimer of twoL-lactic acid molecules, “D-lactide”, which is a dimer of two D-lacticacid molecules and “meso-lactide”, which is a dimer formed from oneL-lactic acid molecule and one D-lactic acid molecule. In addition,50/50 mixtures of L-lactide and D-lactide that have a meltingtemperature of about 126° C. are often referred to as “D,L-lactide”. Anyof these forms of lactide, or mixtures thereof, can be copolymerized inaccordance with this invention. Increased optical purity (i.e., higherconcentrations of D- or L-isomer) tends to cause the resulting polymerto be more crystalline. When a semi-crystalline polymer is desired, itis preferred that the polymer contains either L- or D-lactic acid unitsalone or else contains a mixture of both L- and D-lactic acid units inwhich one of the isomers (either L- or D-) constitutes at most about 3mole %, preferably up to about 2 mole %, more preferably up to about 1.6mole %, and especially up to about 1.2 mole percent of the isomericunits in polymerized form. Particularly preferred semi-crystallinecopolymers contain from 98.4 to 100% L isomer and from 0 to 1.6% Disomer (based on total moles of lactic acid repeating units). When moreamorphous polymers are desired, the ratio of L- and D-isomer repeatingunits in the copolymer is suitably from about 98:2-2:98, preferably from90:10 to 10:90, especially from about 70-90% L-isomers and 10-30% Disomers (based on total moles of lactic acid repeating units).Generally, the selection of stereoisomer ratios will depend on theparticular application and/or desired copolymer properties. In general,the higher the crystallinity, the higher are the thermal performance andthe modulus of the copolymer.

[0039] Certain of the bicyclic monomers may also exist as two or morestereoisomers. An example of this is the2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione described in the examplesbelow. If the bicyclic monomer is not optically pure, the ratio ofstereoisomers may affect crystallinity, and should be taken intoaccount, together with the stereoisomer content of the monomers (i.e.,the monocyclic ester(or corresponding hydroxy acid) and a monocycliccarbonate), in the manufacture of the copolymer, so that desiredproperties are obtained.

[0040] A preferred lactide is produced by polymerizing lactic acid toform a prepolymer, and then depolymerizing the prepolymer andsimultaneously distilling off the lactide that is generated. Such aprocess is described in U.S. Pat. No. 5,274,073 to Gruber et al., whichis incorporated herein by reference.

[0041] When lactide copolymers are to be made, the comonomer isintroduced into the polymerization apparatus. This can be done byblending the comonomer with the monocyclic ester, by adding thecomonomer neat as a separate stream, or by adding the comonomer as asolution in a suitable solvent. Comonomers can be copolymerized randomlyor sequentially to form random and/or block copolymers.

[0042] Another method of preparing the copolymer is to blend thebicyclic monomer with a previously-formed polymer of monocyclic estersand/or carbonate, and then subject the mixture to transesterificationconditions.

[0043] Thermoplastic copolymers of the invention are useful in a varietyof applications, such as fibers (including staple fibers, monofilamentfibers, blended fibers, textured fibers, bicomponent fibers, yarns andthe like), films such as cast film, blown film, oriented film (includingbiaxially oriented film where stretching is performed in two directionseither simultaneously or sequentially), extruded foam, blow molding,compression molding, sheet molding, injection molding, extrusioncoating, paper coating and other applications. In general, the copolymerof the invention can be used in the same applications as thecorresponding homopolymers are used, plus additional applications wherebetter rheological properties are desirable. The copolymer isparticularly useful in applications where excellent shear thinningand/or high melt tension are desirable.

[0044] The copolymers of this invention exhibit improved shear thinningand melt tension compared to linear polymers of the same monocyclicmonomer (at equivalent M_(w)). It is therefore possible to obtain goodprocessing at an equivalent or lower molecular weight than is needed forthe corresponding linear polymers to be processable. This gives theprocessor the option of using lower processing temperatures and/orpressures, thus reducing polymer degradation (monomer reformation,molecular weight loss and color generation), reducing energyconsumption, and in some instances permitting the use of smaller, lessexpensive equipment.

[0045] Of course, the copolymer of the invention can be compounded withadditives of all types, including antioxidants, preservatives, catalystdeactivators, stabilizers, plasticizers, fillers, nucleating agents,colorants of all types and blowing agents. The copolymer may be blendedwith other resins, and laminated or coextruded to other materials toform complex structures.

[0046] The copolymer of this invention can also be blended withadditional amounts of linear polylactic acid polymers to produce ablended polymer having tailored rheological properties. It can also beblended with other polymers, such as polyesters, polyhydroxyalkanoates,polycarbonates, polystyrenics, polyolefins and the like.

[0047] The following examples are provided to illustrate the inventionbut are not intended to limit the scope thereof. All parts andpercentages are by weight unless otherwise indicated.

EXAMPLES 1-5

[0048] A. Preparation of 2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione(bicyclic diester)

[0049] Freshly distilled adipic acid dichloride (100.65 parts) is loadedinto a 3-necked flask equipped with a reflux condenser, dropping funneland stirrer. The adipic acid dichloride is heated to 100° C., and 193.6parts bromine (Br2) is added over about 48 hours with stirring.Volatiles are removed by applying vacuum (20 Torr, 0.5 hour) to yield187 parts of a mixture of meso and d,l-forms of α,α′-dibromoadipic aciddichloride.

[0050] The α,α′-dibromoadipic acid dichloride is added slowly withvigorous stirring to 1750 parts ground ice, so that an emulsion ofhighly-dispersed oil droplets in water is formed. Stirring is continuedfor about 10 hours, during which time the oil droplets form into softpieces and then into a hard powder. The emulsion is then filtered,washed with water and dried in open air. One hundred fifteen parts of amixture of meso and d,l-forms (˜2.0 ratio) of α,α′-dibromoadipic acidhaving a melting point of 185-188° C. are recovered. The water phase isextracted twice with 500 ml diethyl ether. The ether extracts are driedover CaCl₂ and concentrated to yield an additional 46 parts of a mixtureof meso and d,l-forms (˜1.4 ratio) of α,α′-dibromoadipic acid having amelting point of 135-149° C. The separate portions of productα,α′-dibromoadipic acid are combined.

[0051] One hundred parts of the α,α′-dibromoadipic acid in ˜800 partsacetonitrile are charged to a flask equipped with two reflux condensersand a stirrer. The solution is heated to boiling and, with stirring, 35parts of Na₂CO₃ is added over about 1 hour through the top of acondenser. The condenser is periodically washed with a small portion ofacetonitrile. The mixture is then heated to reflux for six hours. Uponsubsequent cooling, a precipitate is formed, which is filtered off. Thefiltrate is evaporated under vacuum (20 Torr) at room temperature. Theresidue is dissolved in 100-150 parts 96% ethanol and the solutionrefrigerated. Approximately 20 parts of product are obtained, which aresublimed under vacuum (90° C., 1-1.5 Torr) to yield 18-19 g ofα,α′-2,5-dioxa-bicyclo[2.2.2]octane-3,6,dione having a meltingtemperature of 136.5° C.

[0052] B. Lactic copolymer polymerization

[0053] Copolymers of α,α′-2,5-dioxa-bicyclo[2.2.2]octane-3,6,dione andL-lactide (1.08% D isomer) are prepared using a polymerization systemincluding three tube reactors and a devolatilizer. Each of the tubereactors is a stainless steel tube 18.5 inches long with an internaldiameter of 2.5 inches containing internal static mixing elements. Eachof the tube reactors is divided into three heated zones. During thecopolymerizations, the tube reactor is heated at 130° C., the second at180° C. and the third at 185° C. The first and second tube reactors areconnected by a transfer line that is held at 180° C. The second andthird tube reactors are connected by a transfer line that is held at200° C. The lactide is introduced into the bottom of the first tubereactor, is transferred from top of the first tube reactor to the bottomof the second tube reactor, is transferred from the top of the secondtube reactor to the bottom of the third tube reactor, and is transferredfrom the top of the third tube reactor to the devolatilizer through aheated transfer line equipped with a divert valve. The devolatilizerconsists of an electrically heated flat plate heater followed by a15″×1″ diameter single-screw extruder which pumps the polymer melt outof a die. The flat plate heater is operated at 250° C. and the extruderis operated at 190-200° C. The unreacted monomers that are removed inthe devolatilizer are trapped in a traced and insulated carbon steel37″×4″ diameter column at 20° C. The entire polymerization system iscomputer controlled.

[0054] The lactide used, if homopolymerized, would be predicted to forma polymer having a M_(n) of 101,000 daltons. The lactide is melted, heldin a feed tank, and fed to the bottom of the first tube reactor using amass flowmeter at a rate of 454 grams/hour. A separate feed of 25% tinoctoate in toluene is pumped into the bottom of the second tube reactorat a rate of 4.38 microliters/minute. Theα,α′-2,5-dioxa-bicyclo[2.2.2]octane-3,6,dione is fed on demand as a 15%solution in gamma-butyrolactone to the bottom of the first tube reactorwith a diaphragm pump using a shot tube. A 25% polyacrylic acid solutionin N-methylpyrrolidinone is pumped into the center zone of the thirdtube reactor at a rate of 13 microliters/minute, in order to kill thecatalyst.

[0055] Control Sample A and Examples 1-5 are made by varying the amountof α,α′-2,5-dioxa-bicyclo[2.2.2]octane-3,6,dione that is fed into thepolymerization system.

[0056] For control Sample A, noα,α′-2,5-dioxa-bicyclo[2.2.2]octane-3,6,dione is added.

[0057] For Examples 1-5, the amount ofα,α′-2,5-dioxa-bicyclo[2.2.2]octane-3,6,dione added is 0.1%, 0.2%, 0.4%,0.4% and 0.6%, respectively, based on the total weight of monomers fed.

[0058] Analysis of the resulting polymers is summarized in the followingtable. Sample/Example No. Property A* 1 2 3 4 5 % bicyclic diester 0 0.10.2 0.4 0.4 0.6 M_(n), PS standard¹ 100,300 86,200 88,300 85,000 90,90091,300 M_(w), PS standard¹ 195,400 171,200 195,900 227,000 251,400287,600 PDI, PS standard¹ 1.95 1.99 2.22 2.67 2.77 3.15 M_(z), PSstandard¹ ND 284,900 370,600 523,400 630,800 854,500 M_(z+1), PSstandard¹ ND 424,000 596,500 950,200 1,279,100 1,833,000 M_(n),Absolute² ND 48,500 51,400 45,900 50,000 50,800 M_(w), Absolute² ND113,600 127,400 170,000 181,200 222,700 PDI, Absolute² ND 2.34 2.48 3.703.62 4.38 M_(z), Absolute² ND 215,400 279,600 498,600 527,800 706,200MFR, g/10 min³ ND 11.1 10.4 6.5 6.6 5.2 Die swell³ ND 1.07 1.19 1.541.52 1.66 Residual lactide, % 8.73** 0.83 0.81 0.63 1.2 1.6 # The mobilephase is tetrahydrofuran pumped at 1 mL/min. The separation is performedon three Waters Styragel HR columns that are connected in series (5micron particle size, 300 mm × 7.8 mm columns with pore sizes of 10⁵Å,10⁴ Å and 10¹ Å). The temperature of the column set and detector is35°C. The detector is a Waters model 410 differential refractometer.Data is analyzed with Millenium 32 software. # The injection volume is0.050 mL. Three PL-gel mixed-B columns (300 × 25 mm, part #1210-6100)are used. The detector is a Viscotek Model 250 DifferentialViscometer/Refractometer. The data is collected and analyzed on apersonal computer running TriSEC GPC software, using universalcalibration with a 3^(rd) order curve fit. Narrow fraction polystyrenestandards from American Polymer Standards Corporation are used toestablish the universal calibration plot. # An average of at least threemeasurements of 1 minute each are used to calculate the melt flow rate.Samples for die swell measurements are collected during the melt flowruns. Approximately 1 inch lengths of molten polymer strand are cut offat the die and cooled. The diameter of the strands is measured anddivided by the known diameter of the die to give melt swell. Reportedresults are an average of at least 5 measurements.

[0059] The data in the foregoing table indicates that branching isintroduced into the copolymer Examples. The M_(n) values (both PSstandard and absolute) remain nearly unchanged as the amount of bicyclicdiester is increased, whereas the M_(w) and higher molecular weightmoments (M_(z) and M_(z+1)) increase significantly with increasing useof the bicyclic diester. Absolute molecular weight measurements showthat these higher molecular weight moments are due in significant partto a high molecular weight shoulder, which increases as the amount ofbicyclic diester is increased. Die swell also increases with increasinguse of the bicyclic diester. Melt flow rates decrease as the bicyclicdiester content increases, but the drop is not precipitous and thereported values are representative of an easily processablethermoplastic.

[0060] The presence of the branches can be inferred from chromatographicmeasurements such as polydispersity. In addition, branching can bequalitatively and quantitatively determined from size exclusionchromatography with multiple detectors. Plotting the log of dilutesolution viscosity against the log of molecular weight (Mark-Houwinkplot) is another appropriate tool for determining branching. In general,polymers having branches will tend to have a lower dilute solutionviscosity, at a given molecular weight, than an otherwise similar linearpolymer. Mark-Houwink plots for copolymer Examples 1, 3 and 5 are givenin FIG. 1. FIG. 1 indicates that, at any given molecular weight, acopolymer made with more bicyclic diester will have a lower dilutesolution viscosity. These data are clear indications that thesecopolymers are branched, and that the high molecular weight shoulder ismainly due to the presence of branched molecules.

[0061] The length of the branches is often inferred from rheologicalmeasurements, and increases in characteristics such as die swell andmelt strength indicate the presence of long-chain branches. Yet anothersuitable method of inferring the existence of long chain branches isdynamic mechanical spectroscopy. Copolymer Examples 1, 2 and 3 are eachdried overnight at 100° C. in a vacuum oven, and placed in a dessicatorcontaining phosphorous pentoxide drying agent. Dynamic mechanicalspectroscopy testing is performed within 8 hours of removing the samplesfrom the oven, using a Rheometrics RDS-2 spectrometer running underRhios 4.4.4 software for machine control and data collection. Specimensare heated to 210° C. and immediately cooled to 180° C. for testing.Samples are run using 25 mm parallel plates, from 100 to 0.01 rad/s at5% strain. The results are shown graphically in FIG. 2. From theseresults, it is seen that as the amount of bicyclic diester increases,the low shear viscosity also increases. All of the copolymers exhibitsignificant shear thinning, but this effect becomes more pronounced asthe amount of bicyclic diester increases. Both of these effects indicatethat the copolymers are long-chain branched.

[0062] The melt tension of copolymer Examples 1, 2, 3 and 5 is evaluatedon a Goettfert test frame. The sample is packed into the capillaryrheometer and extruded at 190° C. at a shear rate of 33 sec⁻¹ through adie 30 mm long and 2 mm in diameter. Melt tension is measured with awheel sensitivity in the range of 1-1000 cN. The wheels of the melttension apparatus are located 110 mm below the capillary die. Resultsare: Example No. % Bicyclic diester Melt Tension (cN) 1 0.1 ˜0.8 2 0.2˜2.0 3 0.4 ˜8.5 5 0.6 ˜13.5

[0063] These results show that melt tension also increases withincreasing bicyclic diester content, and demonstrates how copolymerproperties can be tailored through adjustments in bicyclic diesterlevel. Examples 3 and 5 in particular exhibit excellent melt tensionvalues for a PLA resin.

[0064] C. Blown Film Processing

[0065] Copolymer Example 4 is selected for processing into blown film.The sample is dried in a desiccant drier for 1½ days at 40° C., −40° C.dew point. It is processed into monolayer blown film on a 1″ Killionthree-zone extruder with a 40/80/40 mesh screen pack, through a 3″diameter die with a 0.035″ die gap at a rate of 13 pounds/hour. Extrudertemperatures are 300° F. in zone 1, 345° F. in zone 2 and 365° F. inzone 3, at the clamp and at the die. Haul-off rates are 22 feet/minute.

[0066] The line is started up and run for a period with PLA homopolymerto purge the system, and film is then produced with Copolymer Example 4for about 15 minutes. Bubble stability is excellent and the film hasvery few gels. The film has little to no crystallinity as measured byDSC, but is easily crystallized when oriented by stretching.

[0067] It will be appreciated that many modifications can be made to theinvention as described herein without departing from the spirit of theinvention, the scope of which is defined by the appended claims.

What is claimed is:
 1. A copolymer having, in polymerized form, unitsderived from a (a) monocyclic ester or corresponding hydroxy acid or (b)a monocyclic carbonate, or both (a) and (b), and units derived from abicyclic diester and/or carbonate.
 2. The copolymer of claim 1 whereinthe copolymer contains units derived from a monocyclic ester orcorresponding hydroxy acid, and the monocyclic ester is a lactone, adioxanone, a dioxan(dione), an ester-amide or a mixture of two or moresuch monocyclic esters.
 3. The copolymer of claim 2 which isthermoplastic.
 4. The copolymer of claim 3 wherein the bicyclic diesterand/or carbonate has the structure

wherein each R is independently lower (C₁₋₄) alkyl or hydrogen, each Zis —O— or a covalent bond, each n and each o are independently zero or apositive integer, provided that the values of n and o, taken together,are such that the main ring contains 6 or 7 members when each Z is acovalent bond and 8 or 9 members when each Z is —O—, and Y is—(CR₂)_(m)— where m is 1, 2 or
 3. 5. The copolymer of claim 4, whichcontains from about 0.05 to about 1.5 weight percent, based on the totalweight of the copolymer, of units derived from a bicyclic diester. 6.The copolymer of claim 5, wherein the monocyclic ester is lactide. 7.The copolymer of claim 6 wherein the bicyclic diester is2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione.
 8. The copolymer of claim 7that has a number average molecular weight of from about 10,000 to about500,000, as measured by the GPC/DV method.
 9. The copolymer of claim 8wherein the copolymer is semicrystalline and contains from about 98.4 to99.9 percent of units derived from either the D or L isomer of lacticacid, based on the total moles of the lactic acid units, and from about0.1 to about 1.6 percent of units derived from the other isomer, basedon the total moles of the lactic acid units.
 10. The copolymer of claim9 which contains from about 0.3 to about 1.0 weight percent, based onthe total weight of the copolymer, of repeating units derived from thebicyclic diester.
 11. The copolymer of claim 8 wherein the copolymercontains up to about 98 percent of units derived from either the D or Lisomer of lactic acid, based on the total moles of the lactic acidunits, and about 2 percent or more of units derived from the otherisomer, based on the total moles of the lactic acid units.
 12. Thecopolymer of claim 11 that contains from about 0.3 to about 1.0 weightpercent, based on the total weight of the copolymer, of units derivedfrom the bicyclic diester.
 13. The copolymer of claim 2 which iscrosslinked.
 14. The copolymer of claim 13 wherein the bicyclic diesterand/or carbonate has the structure

wherein each R is independently lower (C₁₋₄) alkyl or hydrogen, each Zis —O— or a covalent bond, each n and each o are independently zero or apositive integer, provided that the values of n and o, taken together,are such that the main ring contains 6 or 7 members when each Z is acovalent bond and 8 or 9 members when each Z is —O—, and Y is—(CR₂)_(m)— where m is 1, 2 or
 3. 15. The copolymer of claim 14, whereinthe copolymer contains units derived from lactide.
 16. The copolymer ofclaim 15 wherein the bicyclic diester and/or carbonate is2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione.
 17. A method comprisingsubjecting a mixture including monocyclic ester and/or carbonate and abicyclic diester and or carbonate to conditions sufficient to polymerizethe mixture to form a copolymer having units derived from the monocyclicester and/or carbonate and repeating units derived from the bicyclicdiester and or carbonate.
 18. The method of claim 17, wherein thebicyclic diester and/or carbonate has the structure

wherein each R is independently lower (C₁₋₄) alkyl or hydrogen, each Zis —O— or a covalent bond, each n and each o are independently zero or apositive integer, provided that the values of n and o, taken together,are such that the main ring contains 6 or 7 members when each Z is acovalent bond and 8 or 9 members when each Z is —O—, and Y is—(CR₂)_(m)— where m is 1, 2 or
 3. 19. The method of claim 18, whereinthe monocyclic ester and/or carbonate is lactide.
 20. The method ofclaim 19, wherein the bicyclic diester and/or carbonate is2,5-dioxa-bicyclo[2.2.2]octane-3,6-dione.
 21. The method of claim 19wherein the copolymer contains at least about 98 weight percent of unitsderived from either the D or L isomer of lactic acid, and up to about 2weight percent of units derived from the other isomer, based on thetotal weight of the lactic acid.
 22. The method of claim 21, wherein thebicyclic diester and/or carbonate constitutes about 0.3 to about 1.0weight percent, based on the total weight of the monomers.
 23. Themethod of claim 19 wherein the copolymer contains no more than about 98weight percent of units derived from either the D or L isomer oflacticacid, and at least about 2 weight percent of units derived from theother isomer, based on the total weight of the lactic acid.
 24. Themethod of claim 23, wherein the bicyclic diester and/or carbonateconstitutes about 0.3 to about 1.0 weight percent, based on the totalweight of the monomers.
 25. The copolymer of claim 6 which has a meltflow rate at 210° C. and under a weight of 2.16 kg of from about 4-12g/10 min and a melt tension of at least about 2 cN.
 26. The copolymer ofclaim 6 which has a melt tension of at least 12 cN.
 27. A methodcomprising melt extruding a film from the copolymer of claim
 1. 28. Themethod of claim 27 that further comprises orienting the film.
 29. Amethod comprising blow molding the copolymer of claim
 1. 30. A methodcomprising melt blending the copolymer of claim 1 with a blowing agent,and melt extruding the blend to form a plastic foam.
 31. A methodcomprising extrusion coating an article with the copolymer of claim 1.32. The copolymer of claim 25 which has a melt tension of at least 6 cN.33. A blend of the copolymer of claim 1 with a linear polylactic acidpolymer.